1. Field of the Invention
The present invention relates to a process of preparing a polymer of a conjugated diene monomer and/or a vinylaromatic hydrocarbon monomer by using a lithium polymerization initiator and the polymer prepared by said method. More particularly, it relates to a process for preparing a polymer having low hysteresis loss and a high molecular weight by using a novel lithium initiator which can be easily used and has excellent efficiency of polymerization initiation and reproducibility and the novel polymer prepared by said process.
2. Description of the Prior Art
The need to reduce fuel cost for automobiles has been increasingly emphasized in recent years. As a result, rubber compounds used for tire treads have been extensively studied with the intent of reducing hysteresis loss. Process for preparing a polymer with a molecular structure having tertiary amines at the ends have also been studied to this end.
One such process is the polymerization of a conjugated diene by using the lithium amides separately prepared in a solvent in advance as the initiator (U.S. Pat. No. 2,849,432). However, because the lithium amides are essentially insoluble or hardly soluble in solvents, storage stability is very poor and storage and handling of the initiator are difficult. When the amide having poor or no solubility is used as the initiator in polymerization, efficiency of the polymerization is low because concentration of the soluble effective initiator necessary for the polymerization is low. Furthermore, the concentration of the initiator is not reproducible and accurate evaluation of the concentration is difficult. Therefore, this process inevitably causes numerous unfavorable results. For example, the molecular weight distribution of the polymer is made broader and reproducibility is bad. Also, the control of structure of the polymer, such as molecular weight, microstructure and the like, by molecular design is made difficult and not reproducible.
Another such process is the polymerization of a conjugated diene and a vinyl aromatic hydrocarbon by using, as the initiator, soluble lithium amides containing a dienyl polymer or the like having an extremely low molecular weight as an additional component and separately prepared in advance (U.S. Pat. No. 5,332,810). When a polymer is industrially produced, it is essential that the initiator used in the production of the polymer does not show any change in properties such as the ability to initiate the polymerization after storage for a long time (storage stability), and that the initiator can be used at any time necessary.
The soluble lithium amide initiator described above, however, shows very inferior storage stability. Because this initiator is inherently unstable as described above, precipitates are formed after storage for only several days, and the ability to initiate the polymerization disappears after storage for a month. When copolymerization, for example copolymerization of styrene and butadiene, is initiated by this initiator, the polymer obtained has a higher molecular weight, a broader molecular weight distribution, a larger Mooney viscosity, and a higher tan .delta., which result in deterioration of the workability and the low hysteresis loss property, the deterioration worsening as the length of time over which the initiator is stored increases.
Still another such process is the polymerization of a conjugated diene in the presence of a solubilizing agent for the lithium amides, such as an ether or the like, by using the lithium amides prepared separately in advance as the initiator (Japanese Patent Application Laid Open Nos. 1975-79590 and 1979-22484). However, since the initiator in this process is also the pre-prepared lithium amides, the same problems as in the process described above, such as insolubility in solvents, poor storage stability and difficulty in handling, still remain in the process of preparing the initiator. Even though the lithium amides show a tendency of being solubilized by the solubilizing agent present in the polymerization system, performance is insufficient. Thus, satisfactory results concerning initiation efficiency, reproducibility of effective initiator concentration and control and reproduction of the molecular structure of the polymer obtained have not been achieved. Furthermore, because ether or the like is used as the solubilizing agent which is the essential component in the polymerization system, content of 1,2-linkage or 3,4-linkage in the diene polymer obtained is increased. Thus, another drawback of this process is that the microstructure of the polymer cannot be varied as desired.
Still another such process is the homopolymerization and copolymerization of an aromatic vinyl compound and a conjugated diolefin, such as styrene and butadiene, wherein the molecular weight and the molecular weight distribution of polymers are controlled by adding a primary or a secondary amine to the polymerization system after the start of the polymerization (Japanese Patent Publication No. Showa 48(1973)-17674).
However, the process described above does not clearly disclose the polymerization conversion which should be reached at the time of the addition of the amine compound after the start of the polymerization. The specification of the aforementioned publication describes that a polymer having a molecular weight distribution in which two peaks are found at a lower molecular weight side and at a higher molecular weight side is obtained when an amine compound is added at a polymerization conversion of about 25 to about 75%. In the examples, an amine compound is added at a polymerization conversion of 37.4 to 77.2%. This manner of addition of an amine compound is natural because the object of the addition of an amine compound in the publication described above is to control molecular weight and molecular weight distribution. Accordingly, a polymer having a low hysteresis loss cannot be obtained by the process of the publication described above. In the present invention, it has been discovered that, when an amine compound is added to the polymerization system at a time when the polymerization conversion is very low or 0%, the polymer obtained exhibits a low hysteresis loss.
A process for preparing a telomer of a 1,3-diene by using butyllithium and a secondary amine is known although it is not a process for preparing a high molecular weight polymer (K. Takebe et al., Tetrahedron Letters, 1972, No. 39, page 4009). This process is related to a process for preparing a telomer containing amine by bringing a large amount of a secondary amine and a small amount of a 1,3-diene into contact with a small amount of butyllithium to effect a continuous transfer reaction between low molecular weight diene lithium and the secondary amine according to the practice of organic synthesis. Thus, this process is entirely different from the process for preparing a polymer of high molecular weight which is the object of the present invention.
The known lithium amides separately prepared in advance which are the initiators have the property of being insoluble or hardly soluble in solvents because they are strong bases containing a hetero chemical elements. Therefore, it has poor storage stability and its ability as the initiator varies according to the time of storage. For example, when the lithium amides are prepared as a solution in a solvent by adding a small amount of an ether compound or a dienyl polymer having a low molecular weight as the solubilizer, the solution changes to a suspended form with the passing of time so as to cause a significant decrease of activity although the system is formed as a solution in the initial stage. This phenomenon is the cause of the numerous unfavorable results as described above regarding the initiator itself and the polymer obtained.
Thus, even though various trials have been made to obtain a polymer having tertiary amines at the ends and low hysteresis loss, no process to date satisfies the requirements of the initiator itself, such as excellent stability, reproducibility, initiating efficiency and easy handling, or the requirements for the polymer obtained, such as the desired molecular structure, reproducibility of the molecular structure and desired physical properties.